A variety of track driven vehicles have been around for many years. Tracked vehicles vary from 100 ton military tanks and bull-dozers to 300 pound snowmobiles. Track types vary from segmented steel tracks to one piece molded rubber tracks.
One of the major design challenges with all types of tracks and vehicles is to find the most efficient way to transfer the torque of the drive mechanism to the track with minimum power loss. There are many torque transmission systems. The three most common torque transmission systems are an external drive, a friction drive and an internal drive. External drives include a sprocket with a fixed number of teeth around the circumference that drives against a rigid member attached to the track. The sprocket teeth protrude through the track to a point where the rigid members can not slip back under a heavy load. Friction drives include a wheel attached to the drive axle and drive against the inside surface of a track. The outside of the wheel and the inside of the track are typically made of resilient material such as rubber or other composites. The track tension must be extremely tight to prevent slippage. The track tension also results in power loss. Internal drive systems, also known as involute drives, have a track with drive lugs attached to the inside surface of the track. The drive lugs may be molded to the inside surface of a rubber track. The drive sprocket is made by attaching rigid drive teeth to a rigid radius wheel. The sprocket teeth drive against the internal drive lugs on the track.
Internal drive systems are generally considered the most efficient drive for tracks made of elastomeric material such as rubber when the drive lugs and drive sprockets are properly matched. They are properly matched when the pitch diameter of the sprocket matches the pitch line of the track. Another way of determining whether they are properly matched is when the pitch diameter of the sprocket causes the drive teeth to match perfectly with the center to center distance between the track drive lugs. In practice, proper matching is difficult to achieve especially when using an elastomeric or rubber track. Tracks made of elastomeric materials are resilient. As a result, the elastomeric material stretches or contracts slightly depending on a number of factors. One of the more common factors that causes changes in the pitch length is the variation in the load applied to a track during operation of the multi-surface vehicle. The load on the track and on the internal lugs will be higher when the vehicle is pulling a log as compared to the load on the track applied to merely move the vehicle over terrain. The tracks may be loaded differently when turning. An outside track will typically be loaded to a higher degree when compared to an inside track. The pitch length of the track varies with the variations in the load applied to the track.
Variations in the pitch length of the track results in a mismatch between the pitch length of the track and the pitch diameter of the sprocket. When using a sprocket having rigid drive teeth, the change in the pitch length along the track causes the sprocket teeth to "scrub in" or "scrub out" or both. In other words, the rigid tooth is rubbing between the individual drive lugs on the internal surface of the flat belt. This causes a loss in efficiency. Scrubbing in or out can result in extreme power loss and excessive wear on the track drive lugs and sprocket teeth.
Another common problem with flat tracks such as those made from an elastomeric material is that foreign matter or sticky material builds up in the sprocket area. Metal tracks usually have openings through which at least some foreign matter may be passed. The buildup is worse on a flat track. When foreign matter builds up in the sprocket area the pitch diameter or the pitch line of the flat track is likely to change. This results in power loss and excessive wear. Rocks, sticks, grass, mud, snow and other materials may build up in the sprocket area.
Military tanks and bull-dozers are two common vehicles featuring metal tracks. Metal tracks are typically mounted on drive wheels and idler wheels that are mounted on springs or suspension systems that allow the drive wheel to move slightly from a fixed position. The use of rollers on the track drive segments of a metal track reduces noise and reduces wear between the individual segments of the metal track. The springs or suspension associated with the idler wheels allows the metal track to accommodate obstacles encountered by the metal track. At the drive wheels, the springs also accommodate slight variations in pitch diameter.
Metal tracked vehicles have many problems. One of the problems is that metal tracked vehicles are very heavy and tend to sink in and damage relatively soft surfaces. The pressure produced by a metal tracked vehicle is relatively high. For example, when a metal tracked vehicle operates in mud, the vehicle typically sinks to solid ground rather than passing over such a surface. The tracks also are tough on surfaces such as grass or lawns. The pressure produced by the metal track of a bull-dozer or a tank typically produces indentations in a surface. For example, if a bull-dozer passes over a residential lawn, the pressure is high enough to compact the earth and form a permanent indentation. A home owner would have to fill in the impressions with additional soil to fix the lawn. In addition, the metal tracks typically have square edges which dig into surfaces during turns. A turning bull-dozer dozer would rec havoc with residential lawns. Metal tracks can also become derailed.
Some tracked vehicles have used rubber tracks. Typically, designers of metal tracked vehicles carry over many of the design characteristics into flat track vehicles using elastomeric or rubber tracks. Many of the problems encountered with metal tracks are also encountered with rubber tracks. For example, many rubber track designs include a track mounted on drive wheels or sprockets which are spring mounted. The problem of matching the pitch line of the track to the pitch diameter of the sprocket is further exacerbated. The drive wheels do not maintain the track near a constant state of tension so the pitch line can fluctuate widely.
In addition, the drive sprocket is positioned so that it in contact with the surface. Typically, the drive sprocket will be at the rear of the vehicle and positioned so that the track passes between the drive wheel and the ground. In such designs, the rear drive wheel has two jobs. The rear drive wheel drives the track and maintains the alignment of the track. When the rear drive wheel is on the ground, the two jobs the rear drive wheel is called on to do work against one another. When driven, the track tends to want to leave the drive wheel or "jump off the sprocket". It is necessary to maintain alignment to prevent derailing. Rear drive wheels on the ground are more prone to derailing since the forces associated with doing the two jobs counteract one another. Another problem with rear drive wheels on the ground is that they tend to require additional complexity. Elongated gear boxes must be used to transfer power to these rear on the ground drive wheels.
Another problem associated with flat elastomeric tracked vehicles is that there are few idler wheels that contact the ground. The track tends to bow between the idler wheels which results in a loss of traction. In addition, with fewer points on the ground and bowing between the wheels, the effective surface pressure at various points under the wheels is high. The tracked vehicle does not have an even pressure across the flat track. Still another problem is that these vehicles are high maintenance. Each individual wheel must be greased periodically. In addition, since the environment for use includes foreign matter such as dirt, the individual idler wheels tend to wear. Because of the high maintenance and cost, there is a tendency to use lesser numbers of wheels in various designs.
As a result of high pressure per wheel, most designs of tracked vehicles using elastomeric or steel tracks are not environmentally friendly. Current designs still indent soft surfaces and tear up grass lands. In addition, the current vehicles are high maintenance. High maintenance is needed to assure that the components of the undercarriage do not prematurely wear.
Thus, there is a need for a for a tracked vehicle that produces a low pressure on the surface and which is environmentally friendly. In addition, there is a need for a lower maintenance vehicle not prone to derailing the track. In addition, there is a need for a vehicle which has many contact points, and therefore has lower pressure per wheel, on the track as it passes over the surface. There is also a need for a vehicle which does not require constant greasing and cleaning of the wheels in contact with the track. There is also a need for a vehicle which places the drive sprocket off the ground so as to eliminate complexity in the design and yet effectively transmit power to the tracks. In addition, there is a need for a sprocket which will accommodate the changes in the pitch line of an elastomeric flat track. In addition, there is a need for a sprocket which will not "scrub" between the driving lugs. There is also a need for a sprocket which is self cleaning and which removes debris from the sprocket area to minimize problems associated with debris build up changing the pitch relationship between the sprocket and the flat track.